1. Field of the Invention
This invention relates to hydrophobic pigments and fillers, their preparation and their use for incorporation in synthetic resins.
2. Background Information
Complete dispersion of pulverulent pigments and fillers in synthetic resins is difficult, especially in foils and sheets, hollow bodies and monofils. Agglomerates of undispersed particles are recognizable as hard lumps or specks and may, in some cases, impair the mechanical properties of the products in which they occur. Incomplete dispersion of pigments causes uneven coloring and the optimum effect of the pigments is usually not fully utilized. It is therefore customary to modify the surface of finely divided solid particles of pigments and fillers by means of substances which render them hydrophobic with a view to improving their dispersibility in synthetic resins.
Polydimethylsiloxanes, for example, may be used for hydrophobicizing various types of surface (see Noll, Chemie und Technologie der Silicone; 2nd Edition 1968, page 386 et seq.)
According to JP-A No. 7 956 083 and JP-A No. 8 143 264, hydrophobic pigments with improved dispersibility are obtained by grinding the finely divided solid particles together with polymethyl hydrogen siloxane.
The pigments obtained by this procedure, however, tend to form a great deal of dust, e.g., in the course of preparation, handling and processing of the pigments.
The formation of dust may be reduced according to DE-A 2 001 469 by using an ester in addition to a siloxane. The esters, however, impair not only the dispersibility, but also the flow properties and ease of dosing the pigments.
Pigments which are free-flowing, easily dosed and with little tendency to form dust may be obtained by subjecting the pigments to a process of granulation.
Thus, for example, a slurry of dry, micronized pigments may be prepared by mixing them with a liquid which can be evaporated, and the slurry may then be dried in a spray drier (GB-A No. 809 284).
The granulated pigments obtained by this method, however, are insufficiently dispersible in synthetic resins, with the result that the mechanical properties of the synthetic resins (e.g., their notched bar impact strength) are seriously impaired.